Carb air screw

[James England]

Hi Folks. I have the 'old style' carbs on my 1979 bike. ie the original XS1100 carbs before they changed them I've been reading (probably too much) about these carbs from various sources but need to verify some conflicting info.

What effect does screwing the mixture srew clockwise (ie screwing it 'in') have? Does it richen the mixture by increasing the amount of fuel going through or does it weaken it.

Just about every carb I've personally known, it richens the mixture when screwed in. Unfortunately, the Haynes manual has nothing about this because we're not supposed to touch them because of emissions (I assume).....

[CatatonicBug]

Yes, that is true. The screw controls the flow of air being mixed with the fuel. Opening (unscrewing) the screw will add more air, leaning the mixture, while closing (screwing in) will reduce air flow, richening the mixture.

[SFerinTEXAS]

Sorry Cat, not so. The idle mixture passage is a fuel passage. Screwing in the screw shuts down that fuel supply.Backing it out increases it . Once backed out so far any additional backing out is irrelevant Fully open is fully open.

[SFerinTEXAS]

Sorry Cat, not so. The idle mixture passage is a fuel passage. Screwing in the screw shuts down that fuel supply.Backing it out increases it . Once backed out so far any additional backing out is irrelevant Fully open is fully open.

To explain further. This passage gets its fuel from the pilot jet circuit. Because it is in front of the butterfly, at closed throttle it feels the vac of the intake much more than the 3 pilot orifices so the effect of the screw interfering with the fuel is greatly magnified thus the effects on idle mixture at closed throttle. When the butterfly's open it becomes just another pilot jet supply port. All the fuel in this passage is mixed with air from the pilot air jet

[TADracer]

And to add to that....

Once your idle mixture screw is so far out it has no effect, if you are still to lean then it is time to go up one step on the pilot jet.

[mack]

Pilot

Wouldn't you change the C clip setting first?

[cywelchjr]

No, the c-clip has no affect at idle, it kicks in during the transition from idle to mains.

[mack]

regret

Ya , as soon as I clicked send I realized my mistake. That happens at 2500 rpm?

[natemoen]

Isn't it somewhere around 4ish turns out where you need to change pilots?

[natemoen]

Ya , as soon as I clicked send I realized my mistake. That happens at 2500 rpm?

2500-3500 ish

[DGXSER]

When to change jets all depends, I know some folks who have run with their mix screws 5 turns out. Some like it under 4 or even 3.

I would agree that if your already 5 turns out and see no improvement or very little, then I would no doubt about it jump the jets up.

[CatatonicBug]

Sorry Cat, not so. The idle mixture passage is a fuel passage. Screwing in the screw shuts down that fuel supply.Backing it out increases it . Once backed out so far any additional backing out is irrelevant Fully open is fully open.

Oops... Thanks for the correction...

[lee_dutcher]

sorry for the hijack... but what if the needle is screwed in to far and the needle tip is jammed inside

[lee_dutcher]

Just about every carb I've personally known, it richens the mixture when screwed in. Unfortunately, the Haynes manual has nothing about this because we're not supposed to touch them because of emissions (I assume).....

lol....................

[cywelchjr]

Ya , as soon as I clicked send I realized my mistake. That happens at 2500 rpm?

Actually it's not really RPM based, but rather airflow based, which while having some relation to RPM, is really more the throttle setting, as if you open the throttles all the way, the slides move up sooner. It's actually IIRC about 1/3 that the mains really start to kick in, and by about 2/3's or so your mostly on the mains. This is at least as I understand how CV carbs work.